Is mononucleosis painful?

Yes. Generally you can have symptoms similar to the flu or a bad cold. You can also have a very severe sore throat with enlarged tonsils.
It can be. Sxs of mononucleosis: fever, sore throat, decreased appetite, feeling tired, feeling poorly overall, rash, swollen lymph nodes, muscle aches & pain & enlarged spleen. Coughing, shortness of breath, headaches, turning yellow, chest discomfort & rapid heart rate may also occur.